In
2013, Angus Campbell caught a 515lb Atlantic bluefin tuna with a rod
and reel off the Isle of Harris, the Outer Hebrides. Although this
wasn’t out of the realms of possibility, this had never been done
before in Scotland. In 2014, Dr. Francis Neat (Marine Scotland
Science) initiated a scientific program on bluefin tuna in Scotland
with the aim of finding out three things: 1) how long bluefin resided
in Scottish waters, 2) where they went, when they left, and 3) what
stock the fish belonged to. That year we successfully tagged three
bluefin with miniPAT tags from Wildlife Computers. Although the
project was a success, the results from our work were far from
conclusive. We’re now in the process of starting a collaborative
study with Stanford University, to contribute knowledge on this
enigmatic species to the bigger picture of bluefin in the northeast
Atlantic.

Bluefin
tuna are commercially important, highly migratory apex predators,
split into three geographically distinct species: Atlantic, Southern
and Pacific bluefin tuna. Demand for these fish has skyrocketed over
the last few decades in line with the rise of the Japanese
sushi-sashimi market, in which bluefin is the most highly prized
delicacy. The majority of bluefin caught is flash frozen and shipped
to Japan for auction, with single fish fetching exorbitant amounts of
money: in 2013 a fish weighting 489lbs sold for $1.76m. Although this
figure was especially high, fish regularly sell for tens of thousands
of dollars.

Atlantic
bluefin tuna are comprised of at least two genetically distinct
stocks, designated by their spawning region: the eastern stock in the
Mediterranean and the western stock in the Gulf of Mexico. Fish from
both stocks make seasonal migrations from warm low-latitude waters to
highly productive foraging grounds at higher latitudes [1]; although
not all fish do this, and whether bluefin migrate or not is related
to maturity and body size, with larger fish ranging further. Despite
being genetically distinct, migratory fish mix extensively outside of
their spawning areas and fish from the western stock can be found in
the eastern Atlantic and vice
versa [2]. As
a result of prolonged overfishing on both sides of the Atlantic, the
western and eastern stocks were reduced to 17% and 33% respectively,
of 1950’s spawning stock biomass by 2008 [3]. This caused the
bluefin regulatory body, ICCAT (the International Commission for the
Conservation of Atlantic Tunas) to introduce stock rebuilding
programs, ultimately resulting in slashed catch quotas.

Atlantic
bluefin tuna, south Donegal, Ireland 2015

The extent of bluefin
distribution is limited by temperature, despite their advanced
thermoregulatory capacity. Recent reports of bluefin in the Greenland
strait (2010), the establishment of small-scale fisheries off Iceland
and Norway (2014), increased sightings off Ireland and Scotland
(2012-13-14), fish caught off Wales (2015) and even sightings off
Cornwall, England (2015) suggests bluefin have repatriated highly
productive northern latitudes in significant numbers in recent years.
This simple fact would lead
us to believe that something has changed; what that ‘something’
is, is cryptic.

Map showing UK and
Ireland Atlantic bluefin tuna sightings 2013-15

We are exploring three
possible causes for these recent changes; 1) a warming ocean climate,
allowing tuna to exploit areas previously too cold, 2) the forage
prey are now ranging further north and in greater abundance than
previously believed, e.g. mackerel, or 3) a recovery of the eastern
bluefin stock, as has been heralded by ICCAT; this would result in a
more significant cohort of larger, more migratory fish. These
hypotheses are not mutually exclusive and may all be acting in
concert. Consequently, this is just the beginning of tuna research in
the UK and Ireland.

Atlantic
bluefin tuna feeding on sprat off Donegal, Ireland 2015

Our work would certainly
not be possible without the efforts of a number of recreational
fishermen, acting responsibly on a catch-and-release basis. This form
of fishing represents a hugely sustainable way of gaining revenue
from bluefin tuna. A
report looking into further developing the existing bluefin
recreational fishery in Canada’s Atlantic provinces, estimated that
1 tonne of bluefin quota allocated to a live-release fishery could
yield up to $100,000; 6 times that of a capture fishery, whilst
having minimal effect on the stock [4]. If the eastern Atlantic
bluefin stock has bounced back, such fisheries may have a place in
the UK and Ireland, and one of the key project aims of our work in
Scotland is to advise as to whether or not this would be a
possibility. Well managed catch-and-release fisheries represent a way
of providing much needed revenue to often remote coastal communities,
whilst also supporting vital scientific research on apex predators
and maintaining good fish stocks.

Tom Horton is a marine biologist, wildlife guide and photographer specialising in the spatial ecology of marine megavertebrates around the UK & Ireland. His current work involves basking sharks, ocean sunfish and Atlantic bluefin tuna. You can follow Tom and check out his work and pictures on Twitter @profhorts.